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USP 51 - Antimicrobial Effectiveness Test (Preservative Challenge Test)
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Summary
USP <51>, also referred to as the Antimicrobial Effectiveness Test (AET), is a preservative challenge test that determines the effectiveness of antimicrobial preservatives in inhibiting microbial growth in pharmaceutical and cosmetic products. The test involves challenging the product with specific bacteria and fungi and measuring microbial reduction at defined time intervals. This confirms whether the preservatives can effectively control microbial contamination, ensuring product safety.
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Quick understanding of the test
USP 51 - Antimicrobial Effectiveness Test
Evaluates the antimicrobial effectiveness of preservatives in pharmaceutical, cosmetic, and personal care products.
- Escherichia coli
- Pseudomonas aeruginosa
- Staphylococcus aureus
- Candida albicans
- Aspergillus niger
- Ensures that preservatives effectively prevent microbial growth.
- Provides reproducible, reliable results for quality assurance.
Turnaround Time
Passing criteria
A significant reduction in microbial counts within the specified time points as outlined by the standard.
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What Is USP 51 Antimicrobial Effectiveness Test?
USP 51, the Antimicrobial Effectiveness Test (AET) is crucial to determine the antimicrobial effectiveness of preservatives in different products including pharmaceutical and personal care products. It outlines a procedure for checking the efficacy of antimicrobial preservatives used to prevent microbial growth that may manifest during production or later during use.
When Is USP 51 Required?
USP 51, the Antimicrobial Effectiveness Test (AET) is crucial to determine the antimicrobial effectiveness of preservatives in different products including pharmaceutical and personal care products. It outlines a procedure for checking the efficacy of antimicrobial preservatives used to prevent microbial growth that may manifest during production or later during use.
Products Applicable for USP 51 Testing
In USP <51> products are grouped into four categories:
- Category 1: Injections, other parenterals, sterile nasal/ophthalmic products with aqueous vehicle
- Category 2: Topical aqueous products, non-sterile nasal & mucous membrane products
- Category 3: Oral products (other than antacids) with aqueous base
- Category 4: Antacids with aqueous base
Test Requirements
Challenge organisms
Candida albicans (ATCC 10231),
Aspergillus niger (ATCC 16404),
Escherichia coli (ATCC 4352),
Pseudomonas aeruginosa (ATCC 9027),
Staphylococcus aureus (ATCC 6538)
USP 51 Test Methodology Overview
- Test microorganisms from stock culture are taken into a sterile vessel and harvested either by centrifugation (for broth medium) or washing (for solid medium).
- These test microorganisms are resuspended in sterile saline and maintained at a microbial count of ~1 X 10^8 CFU/ml.
- The test sample is taken and the initial microbial concentration in it is determined using plate count methods.
- The test sample is then placed in a sterile container and exposed to a specific amount of test organisms.
- Inoculated test samples are incubated at 22.5 ± 2.5°C and the viable count of microorganisms is determined at 7, 14, and 28 days, respectively.
- The preservative effectiveness in the test product is determined by comparing microbial count at specific time intervals to the initial concentration of test microorganisms.
Interpretation of USP 51 Test results
Category | Applicable Products | Log Reduction / Acceptance Criteria |
Category 1 | Injectable products, other parenterals, sterile ophthalmic and nasal products with aqueous bases | Bacteria: ≥ 1.0 log reduction at Day 7 ≥ 3.0 log reduction at Day 14, no increase in count through Day 28 Yeast & Mold: No increase at Day 7, 14, and 28 |
Category 2 | Topical products, non-sterile nasal products, products for mucous membranes | Bacteria: ≥ 2.0 log reduction at Day 14, no increase in count through Day 28 Yeast & Mold: No increase in count at Day 14 and 28 |
Category 3 | Oral products (other than antacids) with aqueous bases | Bacteria: ≥ 1.0 log reduction at Day 14, no increase in count through Day 28 Yeast & Mold: No increase in count at Day 14 and 28 |
Category 4 | Antacids with aqueous bases | Bacteria, Yeast & Mold: No increase in count at Day 14 and 28 |
USP 51 vs Other standards
Parameter | USP <51> (Antimicrobial Effectiveness Test) | ISO 11930 (Preservative Efficacy Test) | PCPC (Personal Care Products Council Method) |
Purpose | To assess the effectiveness of a product’s preservative system in controlling microbial growth in pharmaceutical and cosmetic/personal care products. | To evaluate overall antimicrobial protection and preservative system efficacy of cosmetic products. | To evaluate preservative efficacy during development of personal care and cosmetic products. |
Regulatory Recognition | Compendial method under the United States Pharmacopeia; widely accepted for pharmaceuticals, OTC products, and cosmetics. | Internationally recognized; recommended for cosmetic products under EU/UK regulatory frameworks. | Not legally mandated; commonly used for product development, screening, and internal quality assurance. |
Microorganisms Used | Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, Candida albicans, Aspergillus brasiliensis. | S. aureus, E. coli, P. aeruginosa, C. albicans, A. brasiliensis. | Typically pooled inocula: bacterial pool (S. aureus, P. aeruginosa, E. coli) and fungal pool (Candida albicans, Aspergillus brasiliensis). |
Scope of Application | Aqueous and non-aqueous pharmaceutical dosage forms, cosmetics, and personal care products containing preservatives. | Water-soluble or water-miscible cosmetic products where water is the internal phase. | Broad range of personal care products including creams, lotions, makeup, wipes, eye products, and nonwoven substrates. |
Pass / Acceptance Criteria | Quantitative results expressed as log reductions at defined time points. | Quantitative log reductions; preservative performance classified as Criteria A (robust/strong) or Criteria B (adequate/sufficient). | Quantitative log reduction data; acceptance criteria vary depending on the specific PCPC method used. |
The Importance of USP 51 Testing
Preservative efficacy testing is crucial for preventing potential risks and maintaining the health of consumers. By testing the antimicrobial efficacy of preservative systems, the industry can effectively control product degradation and minimize the adverse effects on consumer health due to microbial contamination.
Why Choose Our Lab for USP 51 Testing
MIS delivers reliable USP 51 preservative efficacy testing backed by experienced microbiologists and transparent result interpretation. Our USP 51 testing service helps pharmaceutical and personal care product manufacturers assess how your formulation performs under real challenge conditions to ensure product safety and efficacy.
Other supporting tests such as USP 61 and Aerobic Plate Count can also be provided.
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Frequently Asked Questions
DR. Martinoz Scholtz
This antimicrobial effectiveness testing outlines the procedures and standards for testing the efficacy of antimicrobial preservatives present in pharmaceuticals, personal care products, and other possible consumer items.
Yes, USP <51> is a type of preservative challenge test
Antimicrobial effectiveness testing is required for any aqueous-based product such as pharmaceuticals, cosmetics, and personal care products.
USP <51> uses five standard challenge organisms: Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, Candida albicans, and Aspergillus brasiliensis.
If a product fails testing, it indicates that the preservative system is not adequate to protect the product against microbial contamination. The product formulation may need to be adjusted, or an alternative preservative system may need to be considered.
USP <51> is primarily intended for pharmaceutical products, while ISO 11930 is designed for cosmetic and personal-care products. ISO 11930 generally has stricter early log-reduction criteria and includes preservation performance classifications (Criteria A and B).
USP <51> typically takes 4-5 weeks.
USP 51 is primarily used for pharmaceuticals, over-the-counter products, personal care products and other regulated items requiring cGMP compliance. PCPC is specifically for cosmetics and personal care products, offering flexibility to accommodate diverse formulations.
This testing is done during the development phase of a product and as a part of stability testing regimen in order to ensure the efficacy of the preservatives throughout the product’s shelf life.
Yes, while USP 51 provides a general framework, the testing can be tailored to the specific characteristics of the product, considering factors like pH, viscosity, and the presence of non-aqueous components.
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